Bone: Distal phalanx
Bones of the human hand
Latin	Phalanx distalis
Gray's	subject #56 107

In human anatomy, the distal or terminal phalanges (singular phalanx) are the terminal limb bones located at the tip of the digits (i.e. fingers and toes). They articulate proximally with the intermediate phalanges at the distal interphalangeal joints (DIP). The distal phalanges carry and shape nails and claws and are therefore occasionally referred to as the ungual phalanges. The distal phalanges are cone-shaped in most mammals, including most primates, but relatively wide and flat in humans.

Human anatomy

Fingers

The distal phalanges of the fingers are convex on their dorsal and flat on their volar surfaces; they are recognized by their small size, and by a roughened, elevated surface of a horseshoe form on the volar surface of the distal extremity of each which serves to support the sensitive pulp of the finger. ¹

In the distal phalanges of the hand the centres for the bodies appear at the distal extremities of the phalanges, instead of at the middle of the bodies, as in the other phalanges. Moreover, of all the bones of the hand, the distal phalanges are the first to ossify. ¹

In the human hand, the distal ends of the distal phalanges possess flat and wide expansions called apical tufts. They serve to support the fleshy pad or pulp on the volar side of the fingertips and the nails on the dorsal side. ²

In the human thumb, the distal phalanx is wider than its proximal neighbour (in contrast to the four distal phalanges of the [ulnar] fingers.)

Toes

The distal phalanges of the toes, in form, resemble those of the fingers; but they are smaller and are flattened from above downward; each presents a broad base for articulation with the corresponding bone of the second row, and an expanded distal extremity for the support of the nail and end of the toe. ³

Evolutionary variation

Skeleton of orangutan, dog, swine, cattle, tapir, and horse

In Neanderthals, the apical tufts were expanded and more robust than in modern and early upper Paleolithic humans. ⁴

In non-human, living primates the apical tufts vary in size, but they are never larger than in humans. Enlarged apical tufts, to the extent they actually reflect expanded digital pulps, may have played a significant role in enhancing friction between the hand and held objects during Neolithic toolmaking. ² Among non-human-primates, phylogeny and style of locomotion appear to play a role in apical tuft size. Suspensory primates and New World monkeys have the smallest apical tufts, while terrestrial quadrupeds and Strepsirrhines have the largest. ⁴

A study of the fingertip morphology of four small-bodied New World monkey species, indicated a correlation between increasing small-branch foraging and reduced flexor and extensor tubercles in distal phalanges and broadened distal parts of distal phalanges. coupled with expanded apical pads and developed epidermal ridges. This suggests that widened distal phalanges were developed in arboreal primates, rather than in quadrupedal terrestrial primates. ⁵

Terminal phalanges of a Masai giraffe

In ungulates, hoofed mammals, the forelimb is optimized for speed and endurance by a combination of length of stride and rapid step; the proximal forelimb segments are short with large muscles, while the distal segments are elongated with less musculature. In two of the major groups of ungulates  — odd-toed and even-toed ungulates — what remain of the "hands" --the metacarpal and phalangeal bones-- are elongated to the extent that they serve little use beyond locomotion. The giraffe, the largest even-toed ungulate, has large terminal phalanges and fused metacarpal bones able to absorb the stress from running. ⁶

Three-fingered sloth

The sloth spend its life hanging upside-down from branches, and has highly specialized third and fourth digits for the purpose. They have short and squat proximal phalanges with much longer terminal phalanges. They have vestigial second and fifth metacarpals, and their palm extends to the distal interphalangeal joints. The arboreal specialization of these terminal phalanges makes it impossible for the sloth to walk on the ground, were the animal has to drag its body with its claws. ⁶

See also

• Bone terminology
• Terms for anatomical location

Notes

References

• Gough-Palmer, Antony L.; Maclachlan, Jody; Routh, Andrew (March 2008). "Paws for Thought: Comparative Radiologic Anatomy of the Mammalian Forelimb". Radiographics (28): 501-10. doi:10.1148/rg.282075061. http://radiographics.rsna.org/content/28/2/501.figures-only.  (PDF)